Running a 5v Arduino at 3.3v on battery

I have researched this and have found multiple answers to this question. Some say that you just change the programming, some say that you just go as is, some say you have to change out the oscillator. So while this is probably one of those discussions like politics or sports (i.e., there is going to be a lot of different opinions) I'd like to get as close to a definitive answer as possible.

I purchased 10 5v boards at $4 ea as the 3.3v version was not available. This isn't all bad since if I ever did want to run them at 16MHz, the option is there, but for now I just want to run them on 2 AA batteries @ 3.3v as they are going to be for simple sensors and would also like to get maximum battery life out of them.

Questions: How is this accomplished? What should I do? If you are able to do this via a step up converter, will you essentially get the same battery life as if you were running a regular 3.3v arduino?

Thanks so much in advance for your help and sorry for the duplication of questions. Just trying to get a fresh perspective instead of piecing together other threads.

@thecricketer Did you read the link or did you not understand it?
It is a highly efficient boost converter which will drain your batteries all the way down to less than a volt while providing 3.3v supply, or 5v with another member of the same family. The original Clone voltage regulators are inefficient (and 5v) so by replacing the VR with this (either 3.3 or 5v) you can suck all energy out of batteries...
Is that not what your question " If you are able to do this via a step up converter, will you essentially get the same battery life as if you were running a regular 3.3v arduino?" was?

@kimot True, 5v has higher power consumption than 3v3 in active operation, but in perspective the higher consumption in an ideal sleep mode (VR and LED removed) is not massive, as link text demonstrated. Modern efficient booster/regulators extend battery life far more than altering fuses accomplished whilst maintaining full processing capability.

3v3 for sure remains the more efficient version (and 5v sensors are less common), incorporating a more efficient supply and maximising sleep time can yield quite spectacular battery life, it really is a question of what lifespan is acceptable.
I don't have your skills or eyesight and bought Nodes pre-made, 3v3, 16MHz, RFM69, down to 0.9v supply, so appreciate what is possible. The Gas meter Node is still on the same 2xAA cells since November last, and registers currently 3.03v...
Having only removed the LEDs on the two clone 5v slaves (5v ultrasonics), had thought to remove the VR and use such a booster, but chickened out. Instead the Nodes power them off when not required, so battery use is minimised.

@sundberg84 True, but unless looking to leading-edge designs the cost is not exorbitant for ready made modules, they are all dropping steadily in price and getting much quieter for noise in each generation.
Ultimately it is a trade-off between frequency of battery replacement and cost, depletion levels, and performance required.
For the moment the stock VR on my 5v Slaves are fine for hourly brief cycles on 4xAA, even were I to use a $1 booster for them from 2xAA they would still be powered off completely between cycles, but the 2 batteries would ultimately be sucked dry as opposed to 4 semi-dead AAs.